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The Effects of Different Aerosol Types and Sizes on Stratospheric Heating

There have been major changes in the Earth’s climate and attempts to mitigate the greenhouse effect have been ineffective thus far. For this reason, geoengineering has been proposed as another method to combat global warming. One common method of geoengineering is through solar radiation management (SRM). One such technique involves injecting aerosol particles into the stratosphere to increase planetary albedo. Studies have show that using SRM in the stratosphere using sulfate particles ultimately cools the planet. Ferraro et al. use a fixed dynamical heating model to examine the affects of various types and sizes of aerosols on stratospheric temperature change. The authors recognize that aerosols heat the tropical lower stratosphere, but can either heat, cool, or not affect the polar regions. They therefore employ various aerosol types and sizes during different seasons in order to further investigate the aerosols’ effect on atmospheric temperature change. The authors conclude that additional research in modeling the impacts of geoengineered aerosols is needed to better understand the effect of this method on stratospheric circulation. —Michela Isono

The majority of SRM research has taken into account the eruption of the Mt. Pinatubo volcano. This eruption increased the sulfate layer produced by the sulfur dioxide emission from the volcano, which ultimately decreased the stratospheric temperature. A change in the stratospheric temperature is also thought to affect the dynamics of the stratosphere; studies have showed irregular weather patterns in various parts of the world such as a warm winter in Northern Europe and a positive phase of the Arctic Oscillation (an index of opposing atmospheric pressure patterns in the Northern middle and high latitude). It is thought that these occurrences were the result of an intensified meridional (in the north-south direction) temperature gradient in the lower stratosphere, which was enhanced by ozone depletion and reduced planetary wave activity.

Ferraro et al. believe that it is necessary to quantify the radiative impact of SRM in the stratosphere before the potential dynamical changes are examined. The size distribution of aerosols is a key factor of which little is known and understood. Previous studies have indicated that the size distribution affects the surface cooling and the stratospheric radiative heating. The authors state that the compositions of aerosols are also significant and propose that soot, limestone dust, and titanium dioxide are viable alternatives. For this reason, the authors investigate the temperature change based on the type and size of aerosols used in order to better understand the impact of SMR in the stratosphere.

Methods

Model: When an aerosol layer is introduced into the stratosphere, the authors use a two-stream radiative transfer code to calculate radiative fluxes (measure of the flow of radiation from a given radioactive source) and heating rates. The temperature change in the stratosphere is calculated using the fixed dynamical heating approximation (FDH). The temperatures are then changed until the stratosphere meets the radiative equilibrium. This method differentiates the radiative impacts from dynamical changes. However, their model does not incorporate the radiative effects on the stratosphere of changing surface temperature. Their model instead shows the activity within the stratosphere before the surface temperature has changed.

Aerosol Layer Properties: When injecting at high altitudes, the amount of time the aerosol resides there is maximized. The authors indicated the existence of technological limits to the input altitude of geoengineering aerosols: plastic balloons burst at about 25 km, and the size of aerosols decrease due to decreasing amalgamation which enhances shortwave (SW) scattering. As a result, the authors introduced the aerosol between the tropopause and 22 km in a standardized layer. It is important to note that the standardized layer is an idealized model because in reality the layer would slope downwards towards the poles.

The authors used aerosols composed of sulfuric acid (sulfate), titanium dioxide (titania), limestone dust, and soot. They also used six size distributions for each type of aerosol, which they characterized as small, medium, and large, and narrow and wide. As a result of the parameters of the aerosols, the authors used Mie calculations (shows scattering of electromagnetic radiation by a sphere) of absorption and scattering. Because radiative properties are a function of aerosol size, the different aerosol distributions will show different radiative forcing (change in net power of electromagnetic radiation per unit area on a surface where a positive forcing warms the system and a negative forcing cools the system). Specifically for this reason, the authors chose the aerosol mass so that the small/wide cases all had instantaneous radiative forcing at the tropopause of –3.5 ± 0.1 Wm–2. Instantaneous radiative forcing measures radiative impact instead of changes in stratospheric temperatures. The authors stated that adjustments in the stratosphere do not significantly impact radiative forcing for most aerosol forms.

Results

The result of the FDH stratospheric temperature adjustment for the small/wide case in the December-January-February (DJF) season and in the June-July-August (JJA) season are compared; the September-October-November and March-April-May seasons are not reported because these seasons include transitions between a warm and cold polar stratosphere. The DJF season and the JJA season showed a very similar patter except with the poles reversed. The mean temperature change in DJF for the small/wide aerosol size distribution for sulfate and soot aerosols showed the same instantaneous radiative forcing at the tropopause. The strong heating in the stratosphere increases downward longwave (LW) emission to the surface, which is why the differences between the instantaneous and stratosphere-adjusted radiative forcings fall under 10% of the instantaneous values, while soot demonstrated a forcing that is 48% less than the instantaneous farcing.

The authors note the importance of understanding the energy balance of a layer in the lower stratosphere when analyzing the data. The stratosphere cools be emitting LW radiation in proportion to the layer’s temperature. They also stated that the main input to the layer is from solar SW radiation. Although some LW radiation from the troposphere goes into the stratosphere, the amount of LW is usually smaller than the incoming SW. As a result, the authors stated that the temperature changes from the aerosols are ruled by LW emission and SW absorption.

The sulfate aerosol showed that the temperature change in the stratosphere correlated strongly with the result of the volcanic natural analogy. There was heating in the tropical lower stratosphere and cooling over the summer pole. The stratosphere is mildly warm over the summer pole, and sulfate emits strongly in the LW and radiative cooling occurs. It was also shown that sulfate absorbs moderately in the LW part of the spectrum. The authors stated the tropical heating was due to the absorption of LW radiation from the warm troposphere and the negligible emission from the cold tropical lower stratosphere.

The titania aerosol showed a temperature change of about 30% of sulfate’s. Heating occurred at all latitudes expect for at the winter pole. This occurrence was due to the fact that titania mainly absorbs the shortest wavelengths, which causes heating in latitudes with solar radiation. The LW cooling takes over, as there is no solar heating because the North Pole is under polar night conditions in DJF.

The limestone aerosol showed patterns similar to titania although they were enhanced; however, limestone showed cooling at lower levels over the South (summer) Pole. The authors noted that the absorption and scattering of incoming solar radiation decreases the radiation accessible for heating at lower latitudes. Therefore, cooling only occurs at the pole because this region is below the aerosol layer. The temperature of the troposphere could not be calculated because the FDH model only applies to the stratosphere.

The soot aerosol showed the most intense heating over the summer pole. This is because soot mainly absorbs SW so its heating pattern in limited by the latitudinal variation of solar radiation. The results showed that soot’s heating is significantly greater than the other aerosol types.

As previously stated, heating always occurs at the tropical lower stratosphere with aerosols, but either heating, cooling or a neutral effect can take place at the poles. Thus, the temperature difference between the Pole and the Equator is affected. The authors use the equation TTR-NP = TTR – TNP to define the difference in temperature between the Tropics (20N–20S) and the North Pole (90N–50N) per unit negative forcing. This is calculated by dividing the temperature change from each model performed by its instantaneous radiative forcing. A positive number indicates cooling at the Pole and warming at the Equator. The authors do not consider the Southern Hemisphere due to redundant characteristics and do not show the large/wide case for titania because the results are irrelevant to SRM.

The results showed that each aerosol type and size distribution increased TTR–NP. IN DJF, titania showed the smallest change in temperature difference (~0.3 K) whereas soot showed the largest change in temperature difference (~2.8 K). Sulfate and limestone both showed an increase in temperature change by approximately 1 K. In JJA, SW absorption heats the North Polar stratosphere. Therefore, sulfate showed a similar temperature change in both seasons, titania and limestone did not show a significant change in temperature, and soot showed a strong negative change (meaning warming at the Pole and cooling at the Equator) of about –5 K.

Results of change in regards to aerosol size distribution were also found. The majority of DJF cases showed that increasing the radius and size distribution increased the temperature change between the Tropics and the North Pole. The large/wide sulfate and limestone aerosol showed a significant change where their mass was ruled by large particles; large particles absorb LW. However, the narrow sulfate case showed negligible affects to changing size, the wide soot case showed negligible affect to radius changes, and the large radius showed the least impact.

In JJA, the temperature change depended on the size distribution of the aerosol. The titania aerosol showed an increased difference in temperature expect for in the small/narrow case (the poles does not cool because LW absorption is low). For this reason, the pole is warmer with this particular titian aerosol. The limestone aerosol also showed a change in the sign: the sign was positive for the small case and negative in the medium and large cases. This is also the result of polar heating, as the larger the limestone aerosol, the greater the absorption of SW radiation. In the JJA season, the pole receives consistent sunlight, which allows more SW radiation to be absorbed, heats the pole, and reduces the temperature difference between the Tropics and the North Pole. In all cases for the soot aerosol, significant SW heating over the poles were shown which decreased the temperature difference.

Discussion

The study showed that different types of aerosols cause different patterns of stratospheric temperature change. The DJF small/wide aerosol types showed the same instantaneous radiative forcing at the tropopause. This means that they all have the same global cooling, however, the atmospheric cooling response may differ from case to case. Stratospheric temperature changes from aerosols result from their rates of SW absorption and LW emission. Aerosols also decrease the temperature of the surface and troposphere, however, because the authors’ model does not calculate temperatures for these regions, the result was not included. The authors focus on the stratospheric temperature changes because surface temperature changes occur over hundreds of years, whereas dynamical changes occur over months to years.

Because the soot aerosol heats the stratosphere intensely, a larger mass of soot is needed per unit radiative forcing despite the results of the calculations without stratospheric adjustment to aerosol heating. The titania aerosol was very responsive to size distributions in regards to its instantaneous radiative forcing- the large/wide aerosol produced a small positive forcing. Therefore, the authors stated that aerosol geoengineering could be ineffective if their thoughts about aerosol size distributions were wrong, which would ultimately alter the nature of regional and planetary responses to SRM.

The authors also compared their results with the yearly variability in the pole-Equator temperature differences. The calculation was based on the standard deviation of the difference in DJF and JJA over the same 20-year period. The yearly standard deviation was for DJF and JJA was 2.09 K and 0.64 K, respectively. To compare those standard deviations with their results, the authors multiplied TTR–NP by the radiative forcing, where the small/wide case was multiplied by 3.5 Wm–2. The calculations showed that in DJF sulfate, limestone, and soot increased the temperature difference more than the standard deviation. In JJA, sulfate increased the temperature difference, and soot decreased the temperature difference past the standard deviation. For the titanita aerosol, no changes in temperature difference were significant in either season.

Conclusion

The results showed strong heating disturbances in the stratosphere. Therefore, it is not adequate to model SRM in the stratosphere by only decreasing the amount of solar irradiance as the stratospheric aerosol heating will not be shown. The authors also conclude that using the same radiative forcing with various aerosol types will not have the same ultimate effect, and that different changes in the stratospheric temperature will lead to various dynamical responses and thus cause the climate to react subjectively. Additionally, the lower stratospheric temperature changes will affect the power of the polar vortex (a big cyclone near both of the earth’s poles in the middle and upper troposphere and the stratosphere). This affect could alter the Arctic Oscillation and various weather patterns. The authors conclude that more dynamical modeling is needed to examine the effect of aerosol radiative absorption on the circulation of the stratosphere and troposphere.